1. Field of the Invention
The present invention relates to materials for a light-emitting element, light-emitting elements, light-emitting devices, electronic devices, and lighting devices.
2. Description of the Related Art
A light-emitting element utilizing a light-emitting organic compound has a structure in which a layer containing the organic compound is interposed between a pair of electrodes. Such a light-emitting element is characterized in that a thin and lightweight light-emitting element can be fabricated, that light is emitted by application of direct current, and that response is fast compared to liquid crystal displays, and the like. Moreover, light-emitting devices in which such light-emitting elements are arranged in matrix form, i.e., passive matrix type light-emitting devices and active matrix type light-emitting devices, are superior to conventional liquid crystal displays in terms of wide viewing angle and excellent visibility. From these reasons, the light-emitting elements are expected to be applied to next-generation flat panel displays. Note that in some cases, these light-emitting elements are referred to as electroluminescent elements or EL elements.
In a light-emitting element, electrons are injected from a cathode into a layer containing an organic compound interposed between a pair of electrodes, and at the same time, holes are injected from an anode into the layer containing the organic compound; thus, the light-emitting element is driven. Electrons injected from the cathode and holes injected from the anode recombine with each other in the layer containing the organic compound to form molecular excitons. The molecular excitons release energy in relaxing to a ground state. Energy at this time may be released as visible light, which can be observed as light emission. Excited states of organic compounds include a singlet excited state and a triplet excited state, and light can be emitted from both of the excited states.
An emission wavelength of a light-emitting element is determined by the energy gap between the ground state and the excited state formed by the recombination, i.e., a band gap. Therefore, light of any color can be obtained by appropriate selection or modification of the molecular structure involved in light emission. Thus, by using tight-emitting elements capable of emitting light of three primary colors of light: red, blue, and green, a full color light-emitting device can be manufactured.
In order to manufacture a high performance full-color light-emitting device, light-emitting elements with long lifetime, high emission efficiency, and excellent color purity which emit red, green, and blue are required. As a result of the recent development of materials, excellent properties of a light-emitting element that emits red light and a light-emitting element that emits green light have been attainable. In contrast, a light-emitting element that emits blue light with sufficient properties has not been obtained. For example, Non-Patent Document 1 reports a light-emitting element that emits blue light, which has a relatively long lifetime; however, sufficient emission efficiency and color purity are not realized.
[Reference]
    [Non-Patent Document]    [Non-Patent Document 1] J. Shi et al., Applied Physics Letters, 2002, Vol. 80, No. 17, pp. 3201-3203